Is recycled concrete green?

I always heard that the process of making cement is very polluting... but is 'wrong' to use recycled concrete in a building?
What happens eventually with this concrete after the house has collapsed, and all the earth and wood go back to the earth. Is concrete a polluting substance?

Re-using old concrete is green because you prevent the dumping of the stuff in landfill. You also prevent the CO2 emissions and adding to the gaping holes in the ground that would be needed to mine new limestone or gravel. So re-use is sure greener than dumping and mining new building materials.

Concrete is not that polluting. Once the setting reaction is over it just weathers like natural rocks would do. Improperly set concrete or cement can seriously change the acidity of the soil and groundwater. I measured pH's of up to 11,5 in dirt because of improperly set concrete.

In some cases polluting stuff has been used as a filling material in the concrete/cement. A filling material is used to reduce the use of expensive cement. Mostly aggregates are used as filler but other stuff has been used as well. Eg Stay clear of concretelike building materials containing slag, cinders or ashes. Even if the bad stuff is not disolving it may cause trouble in the long run - releasing heavy metals in the environment.

If you re-use stuff, spend some tought on what has to happen to remove/recycle your building. One of the mayor problems with recycling building materials is the lack of knowledge concerning origin and composition of the materials used.

Erwin Decoene wrote:Re-using old concrete is green because you prevent the dumping of the stuff in landfill. You also prevent the CO2 emissions and adding to the gaping holes in the ground that would be needed to mine new limestone or gravel. So re-use is sure greener than dumping and mining new building materials.

Concrete is not that polluting. Once the setting reaction is over it just weathers like natural rocks would do. Improperly set concrete or cement can seriously change the acidity of the soil and groundwater. I measured pH's of up to 11,5 in dirt because of improperly set concrete.

In some cases polluting stuff has been used as a filling material in the concrete/cement. A filling material is used to reduce the use of expensive cement. Mostly aggregates are used as filler but other stuff has been used as well. Eg Stay clear of concretelike building materials containing slag, cinders or ashes. Even if the bad stuff is not disolving it may cause trouble in the long run - releasing heavy metals in the environment.

If you re-use stuff, spend some tought on what has to happen to remove/recycle your building. One of the mayor problems with recycling building materials is the lack of knowledge concerning origin and composition of the materials used.

Bedankt voor het uitgebreide antwoord, Erwin:-)

What are 'slag, cinders and ashes'? And how do I recognize them?

Dag Philippe

I will continue in English

https://www.google.be/maps/place/Tournai,+7500+Tournai/@50.5865067,3.4209258,9174m/data=!3m1!1e3!4m2!3m1!1s0x47c2e075ecdff96b:0xd05b7d8bb22073a1?hl=en

This satelite foto shows why reusing concrete instead of fresh concrete is a good idea. The quarries near Tournai are an example of the reason why. Most of those holes has gone up in the air as CO2. CaCO3 + Heat => CaO + CO2. If you want to calculate roughly how much CO2 came out of those holes, calculate the volume - Most are > 150 m deep.
Where not necessary to use new concrete - re-use old stuff. Even better if you can do without concrete or brick.


slag = 'slakken' ~ beware of especially non-ferro slags. In Belgium the regions of Gent-Kanaalzone, Kempen, Plombières, Basecles, lots of others show problems of this kind. Via the link to OVAM and BeNeKempen you can find descriptions to help you along. https://ovam.be/milieu-gezondheid/bodem-saneren-gezondheid/cadmium/benekempen/werkgroep-zinkassen If you buy recycled concrete aggregate with BOFAS-label you should be OK.

cinders = 'sintels' also used in 'cindersteen' There are many local names. Often produced from steam-engine residu left over after burning coal. Can be high in heavy metals etc....

ashes = 'as' or 'verbrandingsas'

Al these products are residu of high temperature burning. To some extent they are glassed (verglaasd) and form a hard substance. Chemically they are not stable. Heavy metals, sulfates etc.... may leach out. 'cindersteen' can even burst apart suddenly due to chemical wethering. New minerals, more voluminous minerals form inside the stone and make it burst suddenly. This stuff shows up in recycled materials sometimes - avoid when possible.

You should not want to have them in or near your garden. The BeNeKempen - reports on aggriculture show why.



If you go for re-use of steelslag - 'hoogovenslak' take in account that one day you might want to remove it. Some types of steelslag come with a 'gebruikscertificaat' or some other official document legalizing its use. HOWEVER there are no takers for the product once you bring it to your local containerpark. So suddenly your cheap product becomes very expensive to get rid of. If you use steelslag - use only the types that are certified AND MAKE SURE YOU USE THEM ON TOP OF GEOTEXTILE. That way you can remove them unmixed with other stuff. Near Liege lots of Walloon steelslag is spread out over Haspengouw. Of course that came without certificate. Even the stuf that comes with a certificate leaches sulfates and such.

I would not use it anywhere outside an industrial site.

The most polluted concrete that I have dealt with, was from the floor of an old automobile service station. Lots of spilled oil had been absorbed.

Battery and radiator shops and other places dealing with chemicals can be problematic.

Dock yards deal with an array of nasty chemicals.

Scrap metal yards become so polluted that all concrete and soil should be considered toxic.

I have made use of old concrete. Sidewalks tend to be pretty clean, and they break up into chunks that one man can lift, and that are generally free of rebar.

Since most have no control or knowledge of the materials and manufacturing process, since it is the factories that take on the legal liabilities, I'd say using recycled concrete is not a good idea. Here in the USA in foundations, it is next to impossible in the most expensive parts of a building the foundation due to code, we have no compression or flexure allowable to design to I am aware of and many foundations struggle at minimum code with consistant density and/or mechanical properties. Without those monolithic properties both mechanical and physical issues can arise, that would take a lab and pro to determine based on many variables. That makes all else a moot point if the building can't take the structural loads. We could get into it's embodied energy but I think in some cases like lime manufacturing, and many others, are not far less of a kiln temp it may be higher than portland cement(OPC) when you consider shipping. Here we only use 10-12% OPC, fly ash is an option to take it down if you do not mind the less than 1% heavy metals which do not bother me since it is recycled off kiln walls as a byproduct. The steel rebar is of most concern to me due to rust jacking but I replace it with FRPs. Magnesium, other pozzolans, geotec cements make it more natural but that is probably best designed by qualified professionals, and as I said above that take on the professional liability.

Dale, we have new natural sealers coming to market and they can be polished for industrial apps. 1200 grit with the right sealer makes them practically impermeable to liquids but breathable to vapors. Some silcone/ silicate sealers actually improve the permeability to 125%, manage efflorescence and freeze-thaw damage New natural densifers are helping too.

Thank you very much for your clear answers Erwin, Dale and Terry!
I will probably try to avoid using concrete where possible, but I still have a lot of research to do on alternative methods for foundation. The house we want to build consists of several small (diameter 6m) roundwood buildings. The roof (ideally a living roof, which would make it a lot heavier) would be supported completely by roundwood posts. Non-structural walls would be cob and strawbale.

I'm not sure about foundations, but I have some really cool retaining walls/ beds made of recycled concrete. I even got a bunch of iron oxide stained ones, which really looks like stone.

Gilbert Fritz wrote:I'm not sure about foundations, but I have some really cool retaining walls/ beds made of recycled concrete. I even got a bunch of iron oxide stained ones, which really looks like stone.

Sounds cool! Do you have pictures?